Electronic devices such as semiconductor integrated circuits, electronic flat panel displays, solar cells, or solid state light emitting devices generally are manufactured by performing a sequence of process steps in various plasma chambers. Such process steps commonly include steps performed within plasma chambers, such as an etch step performed in a plasma etch chamber or a deposition step performed in a plasma-enhanced chemical vapor deposition chamber or a plasma sputtering chamber. Any chamber for performing a step in the manufacture of an electronic device while a plasma exists within the chamber is considered a plasma chamber.
Malfunctions occasionally occur to components within a plasma chamber. If a plasma process is not halted after a malfunction occurs, there is a risk of producing more serious damage to the chamber components or the workpiece. Therefore, it is important to detect such malfunctions as quickly as possible so that the plasma process can be halted.
One common malfunction is broken ground straps attached to the susceptor that holds the workpiece. In many plasma chambers, the susceptor is moved to a lower position when a workpiece is moved into or out of the chamber and is moved to an upper position while plasma process are being performed on the workpiece. The up and down movement of the susceptor flexes and eventually breaks some of the ground straps attached to the susceptor.
Another common malfunction is cracking or breakage of the workpiece itself. Such damage to the workpiece is most likely to occur at the start of a plasma process because the workpiece is subjected to rapid changes in temperature and voltage at that time.
US patent application publication no. 2008-0074255 A1, having the same assignee as the present application, discloses an apparatus capable of detecting such malfunctions when RF power is supplied to the plasma chamber by a variable-frequency RF power supply whose frequency is adjusted to optimize the impedance match between its output and the load, so that the frequency of the RF power supply changes in response to changes in the load impedance presented by the plasma chamber. In that design, upon a change in the state of the workpiece processing, such as when plasma processing of a new workpiece begins, the RF power supply frequency is compared with lower and upper frequency limits. A malfunction is detected if the RF power supply frequency moves outside the “normal” frequency range bounded by the lower and upper frequency limits. There is no disclosure of adaptively updating the frequency limits as a function of the power supply frequency range actually experienced during processing of the previous workpiece.